Launching an unprovoked attack upon the Federation, the Klingons have broken the terms of the Organian Peace Treaty – leaving the Enterprise stranded deep in uncharted space, cut off from the rest of the Federation fleet.

To find out what happened to the Organians, CommanderScott rigged an experimental modification of our transportation system designed to enable a tachyon replica of Mr. Spock to travel faster-than-light to Organia. But the experiment failed, and incredibly, left us with two identical Spocks: one of whom is a replica in every way the opposite of our first officer, a traitor whose continued existence poses a grave threat to the Enterprise and quite possibly the entire Federation.

The Bantam novel series have a plethora of different cover images: the English version of this novel has been released with at least three different covers, a new one for each subsequent printing. There have been three US cover printings by Bantam (February 1970, April 1985, and June 1999). The 1990s edition cover is credited to artist Kazuhiko Sano.

An editor at Bantam copyedited Kirk's reference to McCoy from "Bones" to "Doc", apparently being unfamiliar with the reference to McCoy's oft-used on-screen nickname. Author James Blish wrote a piece about the error in the introduction of a later novelization.

Canon characters listed below are linked to the main article about them. Non-canon characters are not linked, but those that recurred, appearing or being mentioned in more than one story, are defined further in Bantam TOS characters.